Engine misfire occurs when charge delivered to an engine's combustion chamber fails to ignite. Misfire can be a symptom of unstable engine operating conditions and can cause inefficient engine operation, reduced engine power and performance, and high emissions of unburned fuel. Accordingly, it is desirable to detect engine misfire so that it can be diagnosed and so that steps can be taken to prevent it from re-occurring. There can be a number of different causes for engine misfire, such as reduced compression, which can be caused by worn piston rings, a failed head gasket, a worn crankshaft, intake or exhaust valves that are not seating properly. Another cause for engine misfire relates to the fuel injection system, for example, if the fuel injector is broken or not working properly to introduce the required amount of fuel. If the engine uses exhaust gas recirculation, the valve that controls the amount of exhaust gas that is recirculated could be broken or stuck, so that too much exhaust gas is recirculated. If an engine uses oxygen sensors or mass flow sensors to control the fuel air mixture the failure of one of these sensors could cause misfire. Engines that use an ignition assist, such as a spark-plug in Otto Cycle engines or a glow plug in Diesel Cycle engines, could misfire if the ignition assist device or the controls for timing or energizing the ignition assist device fails. These are only some examples of the causes for engine misfire, which illustrate the need to detect it so that corrective action can be taken. Depending upon the cause, this corrective action could be an adjustment to the operating parameters made by the engine's electronic control unit or the corrective action could involve a service inspection to determine the cause of the misfiring and maintenance service to repair or replace engine components.
Accelerometer sensors are sold commercially for use as knock sensors for detecting engine knock. Engine knock correlates to violent combustion events that can be caused by pre-mature and uncontrolled detonation of the charge inside the combustion chamber, caused, for example, if an Otto-Cycle engine has been fuelled with a fuel with too low an octane rating, or if spark timing is too advanced, or if deposits in a combustion chamber create hot spots that cause early ignition. In a Diesel-Cycle engine, engine knock can be caused, for example, if fuel injection timing is too early. Engine knock can cause a decrease in engine performance and in severe cases, if not corrected, engine knock can cause serious damage to the engine, including destruction of the pistons, connecting rods, exhaust valves, head gasket and spark plugs or glow plugs. Accelerometers that are employed as “knock sensors” are typically located on an engine's cylinder block and sometimes on the cylinder head. Wide-band knock sensors are simple transducers that transmit a signal to a knock processor chip that detects engine knock from the sensor signal. The knock processor chip sends a signal to the electronic engine controller when engine knock is detected. There are also knock sensors that transmit a binary signal when an element within them vibrates at a harmonic frequency that matches the knocking frequencies of the engine. Knock sensors of either type are only required to make crude measurements since they are configured to detect only engine knock and the difference between the signals when the engine is not knocking and when the engine is at a defined knocking level is sufficiently large to not require sensors with a high degree of accuracy. Wide-band knock sensors transmit signals that are proportional to frequencies that include vibrations in the knocking range, and hereinafter reference to knock sensors and accelerometer sensors are directed to wide-band knock sensors.
Recent developments have been made to show that by mounting an accelerometer sensor where there is a higher signal-to-noise ratio, accelerometer sensors can be used to detect deflections caused by changes in the in-cylinder pressure during combustion events that have much lower amplitudes than the deflections caused by engine knock. The component to which the accelerometer is attached can act like a damped oscillator. The accelerometer signal can be filtered to reduce the effects of noise so that the accelerometer signal can be used to determine information about engine combustion characteristics such as, for example, the timing for start of combustion, the heat release rate, and the peak heat release rate. However, from such an accelerometer signal it can be difficult to detect engine misfire under all operating conditions. At high load conditions, the amplitude of the accelerometer signal increases significantly when combustion occurs and when the expected rise in amplitude does not occur this is a good indication that misfire has occurred. However, at low load conditions and when the engine is idling, the amplitude of the deflections of the engine component where the accelerometer sensor is installed is much less compared to when under high load conditions, so the amplitude of the accelerometer signal is also lower. With lower signal amplitudes associated with combustion events under low load and idle conditions it can be difficult to detect misfire if there is signal noise in the accelerometer signal. Signal noise is unavoidable because the component to which the accelerometer sensor is mounted is also deflected by other effects such as, for example, engine vibration, road “noise” caused by uneven road conditions when the engine is installed in a vehicle, and deflections caused by the operation of other mechanical devices attached to the engine. While some signal noise can be filtered out, it can still be difficult to detect misfire, or the absence of combustion, since some signal noise inevitably remains in the filtered signal, and the amplitude of the signal noise can be difficult to distinguish from the signal component that is caused by the engine firing. Accordingly, there is a need for a robust and accurate method of using an accelerometer signal to detect misfire.